fix: add isAvailable check in getInvoker to prevent idle eviction race condition

Author: Phoebe0126

trpc-core/src/main/java/com/tencent/trpc/core/cluster/def/DefClusterInvoker.java

@@ -72,7 +72,14 @@ protected CompletionStage<Response> doInvoke(Request request, CompletionStage<Se
     protected ConsumerInvokerProxy<T> getInvoker(ServiceInstance instance) {
         String key = toUniqKey(instance);
         ConsumerInvokerProxy<T> result = invokerCache.get(key);
-        return Optional.ofNullable(result).orElseGet(() -> createInvoker(instance));
+        // Check availability on fast path to prevent using an invoker whose underlying
+        // RpcClient is being closed by the idle scanner (scanUnusedClient).
+        // Without this check, a race condition exists between the scheduler closing the
+        // client and the closeFuture callback removing the invoker from cache.
+        if (result != null && result.isAvailable()) {
+            return result;
+        }
+        return createInvoker(instance);
     }
 
     @SuppressWarnings("rawtypes")